Sigma 150-500 for wildlife?I thought about the 80-400, but at the end of the day it is about $400 more expensive (new, though a quick look at Amazon shows some used for as cheap as $800), less long end range, and only slightly faster. If 300mm is a long enough range that leaves me open to several cheaper options, some under $500, though I kind of think I'd like to have the 500mm, even if I rarely use it, just in case?